Figure 2: A comparison of experimental heat flow (solid red curve) with the calculations performed using Equation (2).

It is seen that Eq. (2) gives a very good reproduction of all basic thermal effects—structural relaxation-induced heat release below Tg, heat absorption above Tg and the first crystallization-induced heat release above Tx. According to Eq. (2), these heat effects are controlled by relaxation of the shear modulus. The underlying physical reason can be understood as relaxation in the system of internal “defects”, which can be identified as dumbbell interstitials or elastic dipoles.